1. Field of the Invention
This invention relates to rubber compositions suitable for use particularly in automobile tires.
2. Description of the Prior Art
Rubber compositions of a high degree of hardness are in common use for belt and bead filler portions of automotive tires. To attain improved hardness, attempts have been made to blend a given starting rubber with large amounts of carbon black. Too much carbon black, say greater than 50 parts by weight based on 100 parts by weight of the starting rubber, however, will in most cases render the finished composition mechanically weak for example with respect to tensile strength on break. As is generally known in the art, this problem can be alleviated to some extent by the use of a low structure carbon black not abundant in dibutyl phthalate adsorptivity. To this end, N326 is oftentimes employed which is a carbon black of a lower structure type than a standard N330 type among high abrasion furnace blacks (HAF). Various N326 carbon blacks have been proposed as disclosed for instance in Japanese Laid-Open Publication Nos. 50-60904, 59-89339 and 60-4540.
Prior high-hardness rubber compositions leave their mechanical strength characteristics unsolved. These compositions when formulated to provide a dynamic modulus greater than 6.0 MPa involve too small tensile strength to meet the commercial requirements.